Centrifugal compressor



2 Sheets-Sheet 1 Filed Dec. 22, 1950 Gttornegs Jan, 7, 1958 R. P.ATKINSON CENTRIF'UGAL COMPRESSOR Filed Dec. 22, 1950 ENT VELOClTY A ICDENSITY 2 Sheecs--ShLaMl 2 United States Patent O 2,819,012 CENTRIFUGALCOMPRESSOR Robert P. Atkinson, Indianapolis, Ind., assignor to GeneralMotors Corporation, Detroit, Mich., a corporation of DelawareApplication December 22, 1950, Serial No. 202,273

11 Claims. (Cl. 2313-119) My invention relatos to centrifugalcompressors, more particularly to compressors of aknown type' in whichair enters the compressor parallel to its axis of rotation, engages aninducer portion of the compressor rotor provided with generally helicalblading in which the air ows in a predominantly axial direction and isaccelerated tangentially, and then proceeds through an impeller portionof the rotor formed with substantially radial vanes in which the air isaccelerated tangentially and radially, leaving the rotor with a hightangential velocity. In such a compressor, the air discharged from therotor is received in a diffuser in which the Velocity head of the air islargely converted to static head and the air is directed to the outletor outlets of the compressor. It is to beunderstood, however, that thisinvention is concerned not with the diffuser as such but rather with theform of the rotor.

The invention is particularly intended for and highly valuable in highperformance centrifugal compressors such as are used in gas turbineengines. lt is well known that compressors for this service ordinarilymust handle a large volume of air, must operate at a high compressionratio, and must be of high efficiency. The adoption of the principles ofthe invention in a previously known type of gas turbine engine hasresulted in remarkable increases in the power and eliiciency of theengine.

rThe principal objects of the invention are to provide a centrifugalcompressor of high capacity, one of high compression ratio, and one ofhigh efficiency. Further objects of the invention are to increase theflexibility and stability of centrifugal compressors and to improve theperformance of such compressors and of motive power systems embodyingcompressors. A further object of the invention is to extend the range ofusefulness of centrifugal compressors, especially single stagecompressors, and thereby render more advantageous the use of therelatively simple and reliable centrifugal compressor in gas turbineengines, where heretofore the relatively temperamental, expensive,complicated, and delicate axial-110W compressor has been considered tobe more suitable.

Expressed in another way, the objects of the invention may be defined asto provide a centrifugal compressor in which the ow path delined by thestructure of the compressor is best accommodated to the natural path offlow of the gaseous medium, in which the distribution of pressure at anysection of the passages of the compressor is as uniform as practicable,in which velocity distribution is uniform across the passages to a muchgreater extent than hitherto, and in which the tendency to supersonicflow and choking is suppressed to a greater extent and over a widerrange of operating conditions than hitherto considered possible.

The principles ofthe invention and the manner in which they are appliedto obtain the highly important advantages of the invention are difficultto summarize, but will be apparent to those skilled in the art from thesubsequent detailed description of the principles of the invention andet a compressor embodying those principles.

2,319,912 .'lfatented Jan. 7, 195:@

Referring to the drawings: Figure 1 is a diagrammatic sectional view ofa single stage single entry radial flow compressor, taken on a planecontaining the axis of rotation; Figure 2 is a diagram illustrating theblade thickness; Figure 3 is a diagram illustrating the preferred formof inducer blade; Figure 4 is a vector diagram illustrating conditionsat the compressor inlet; Figure 5 is a vector diagram illustrative offlow conditions within the rotor; Figure 6 is a chart illustrating thevariation of static density, component velocity, and net area of thepath through the rotor; Figure 7 is` a diagram expository of theprinciples of the invention; Figure 8 is: a diagram illustrating theconstruction of the mean flow path or centroid ofthe rotor; and Figure 9is a diagram illustrating the construction of the boundaries of the flowpath.

Although 'compressors of the general type to which the invention relatesare well known, it is believed advisable to describe briefly thestructure of such a compressor in order to eliminate any possiblemisunderstanding as to terminology in the specification. Referring toFigure l, the compressor illustrated diagrammatically comprises a rotor10 mounted on a shaft 11 for rotation therewith. The rotor comprises aninducer portion l?. and an impeller portion 13. While these may beintegral, they are ordinarily made separately for manufacturing reasonsand are fixed together. The inducer abuts the impeller at a radial planeindicated at 14 and cornmonly called the split line. The shaft 11 ismounted in bearings indicated by bushings 16 and 17 in the xed structureof the compressor. The forward bearing 16 is mounted in a body 18 whichdefines the inner boundary of the compressor inlet 19. `An annularcasing 21 defines the outer boundary of the inlet and of the liow paththrough the rotor 10. The frame member 22 which supports the rearbearing 17 may be a disk closely adjacent the rear face of impeller 13.The inducer 12 is fitted with radial blades 23 which are of curved formto accelerate the air tangentially, as will be described more fully. Theimpeller `13 is provided with radial blades 24 which ordinarilyconstitute continuations of the impeller blades 23, a continuous passage25 for flow of air from the compressor inlet 19 to the compressor outlet26 being defined between each adjacent set of rotor and impeller blades.Forl greater clarity, one complete blade 23, 24 is illustrated as viewedat right angles to the plane of the impeller blade 24 in Figure 1. Aswill be apparent, `the body of the rotor. constitutes the inner boundaryof the air flow path through the rotor. The outer boundary could ofcourse be defined by a shroud fixed to the rotor but may be defined bythe fixed casing structure 21 adjacent the tips of the blades, asillustrated. The air discharged from the rotor enters a diffuser 28which may be of any suitable form, and is illustrated schematically asprovided with a plurality of discharge outlets 29.

Although Figure 1 illustrates la single entry compressor, the principleof the invention is equally applicable to double entry and tomulti-stage compressors. The form of the inlet mayA also be varied tosuit the particular installation. The impeller blades `are ordinarilyradial, but may be curved ahead or back.

The nature of the invention may be indicated by stating that it involvesproviding a ilow path in the compressor conforming to the natural flowpath of the air or other liuid (referred to as air for conciseness).

The air entering the compressor is considered to have a known velocityparallel to the axis of rotation of the compressor, without radial orcircumferential components.

In the inducer portion of the rotor, the air is acceleratedcircumferentially of the axis, or tangentially, in as uniform amanner'as practicable, by the inducer vanes. As the Vair acquires`ataugential velocity, centrifugal force aai/aora' 13 acts upon the airin a radially outward direction. In the impeller, the angular velocityof the `air about the axis remains constant, but Ithe centrifugal forceincreases with the radius. There is thus a continuously increasingcentrifugal force on a lparticle of fair as it :traverses the rotor frominletto-outlet.

The `inertia of the air resists `the radial acceleration, and a particleor' :air thus .tends to follow a curved path, relative to the rotor as aframe .of reference, under the action of the initial or entrancevelocity and ,the centrifugal force -exertedonthe particle.

The problem of ,providing an .optimum ow path is complicated by thefactthat .thefaircannot 4be `treated as a free particle under `the action of.centrifugal and @inertia forces, due to the effects ,of .frictiom tot.back pressure in the rotor outlet, and .of `variations in ,the densityofthe air and the areasof-.the ilowipath. A

The rotor ,ofthe invention Yis formed Iso ythat the flow path providedfor .the air conforms rto lthe path .of -frec movement 'due to theentrance velocity .and centrifugal force with allowance lfor the`various .modifying `vfactors just described. As `a result, the flow vis-not forced .from its natural path bythe physical boundaries of thechannel defined by the compressor casing (or shroud, if present) and thesurface of therotor.

The .mean ,path :is determined for a yprescribed relation betweencomponent velocity (velocity in the ,plane containing theaxis) anddistance along the -meanpath The area of the passage is modified along.the .length `of Vthe passage to v.provide relation.

As will be apparent, .the tangential velocity yand the radialacceleration increase as the air moves radially outward through theimpeller `portion .of the rotor, .and

the final path of vthe Vair -(apart from the tangential component)approaches alradial direction. The air is discharged with .a highabsolute velocity, the .major component of which .is tangential, andIthe velocity -head is largely converted to static pressure in .thediffuser, which may be of known type.

The principles yot :the invention `may be most clearly explained bydisclosing 4the Aprocedure involved in designing a Acompressor accordinguto the invention to ymeet given design conditions. Obviously, `the size.and `speciiic form of the compressor will vary with Lsuch lparametersas diameter, speed, air .flow pressure, and the like, but suchvariations Vdo not affect the `'design procedure.

The following -basic design conditions must be established or assumed:(l) pressure and ltemperature of air at the inlet, (2) air flow desired,weight per unit time, (3') rotor speed, revolutions per unit time, (4)lassumed etiicicncy, and ('5) assumed vpressure ratio (total outletpressure to total inlet pnessure). O'f these items, l), (2), (3), and(5) depend upon the installation for which the compressor is built. Forva gas turbine engine, `pressure and temperature of `the air .may beassumed atmospheric conditions. The ,desired air liow would 'bedetermined by the engine air requirements. The rotor speed is usuallythat of the turbine, ,and is limited by the necessity to avoid excessivetangential velocities, Yand thereby by the diameter of the rotor. Thevalue assumed for eiliciency is based upon experience', and 'for vthe`compressor of the invention experience indicates a'value oi 80% to besuitable for designputposcs. The vassumed pressure ratio is determinedbythe rotortipspeed'niof course, must not exceed whatfcan be realized,'The values assumed must be compatible, `and the selection -of designparameters may involve trial and error 'to some extent.

Certain dimensional values must also b'e' assumed: (l) impeller radius,('2) outer and inner radiioff the annular inlet, (3) impeller bladewidth at the outlet, Y(4,) axial length of the inducer portion, (5)number of `blades, and (6) blade thickness, normally f expressed interms lof thickness at the 4tip and at the rootof the blade.

The impeller radius isordinarilyflimited by the dimen` vtheaibovementioned prescribed 4 sions of `the engine. It is related tospeed, as excessive centrifugal forces in the rotor must be avoided.-'Pressure ratio is determined primarily by the speed and diameter ofthe rotor.

The inlet diameter is based on a compromise between having a largediameter to maintain a reasonably low inlet velocity and a smalldiameter to reduce `the relative Mach number at :the outer diameterofthe inlet eye.

The outlet width of the impeller is based in the usual manner on the dowconditions .at the outlet, that is, .upon the area necessary lfor iiowIof the assumed weightof air and lassumed outlet conditions. it is notcritical, and may be varied over a rather -wide vrange to suit.installation requirements.

The length of the `induceiumay `be limited by the allowable overalllength of the rotor and machine tool require ments. in someinstallations, especially double-sided compressors and cases i-n which a.compressor is substituted in a previously existing engine, it .may not`be possible to malte the inducer of optimum length. 'In the `inducer,the tangential velocity of the .air is increased at roughly constantradius. In the impeller, the .tangential velocity is increased as theair flows outward, tangential velocity being proportional to the radius.There is a gradual 'transition from axial iiow at the inducer entranceto nearly .radial ilow at the rimpeller outlet. As indicated -above,installation conditions may dictate `an inducer shorter than optimum,but, in general, an etiort should be made to provide an 'inducer portionof such length that tangential acceleration Vin the inducerapproximately equals that in the impeller.

The number of blades is a matter of informed choice vbased upon 'testand experience. With too few blades,

there is excessive turbulence; with too many blades, excessive friction.A small number of blades will give rise to Igreater pressureliuctuations The use of a prime number of blades may reduce resonance.

The blade thickness Tis a matter of structural design to obtainsuliicient mechanical strength combined with the required vibrationcharacteristics .and a structure adapted to manufacturing processes.Figure 2 illustrates blade thickness in the Vimpeller, the thickness atthe tip of the lblade being indicated by aand at the root of the bladeby b.

As Willlbe apparent 'to those skilledin the art, the above discussion ofdesign procedure largely relates to background material, as these orequivalent assumptions are necessary for the design of compressorsingeneral .of the type to Vwhich the invention relates. vFor thisreason, the computations and the underlying theory .are not discussed indetail.

From the design conditions and physical dimensions assumed, the area,velocity of ow, static density, total pressure, and total temperature,at the rotor inlet and outlet, may be computed in'known manner.

The `preferred inducer blading, upon which the subsequent discussion isbased, is ysuch that the .line of intersection of a blade with any planeperpendicular to the axis is radial, and the intersection of a blade bya cylinder concentric with the axis is a parabolic curve inthe cylinder. vOtherwise expressed, the blade is of the form, in cylindricalcoordinates, T is proportional to X2, where X is the distance measuredalong the axis from vthe discharge end of the inducer and T is theangular displacement of a radial 'blade element relative to thedischarge end of 'the blade. At any given radius, we may state Vthis asY=KX2, where Y is the length of arc through which the blade isdisplaced, .K is a constant to be determined, and X is as before. Thisrelation is illustrated in Figure 3. The first step in the determinationof the constant K is the determination of the angle of attack of theinducer blade at the centroid ofthe inlet passage. VIn thisspecilication, the term centroid is defined as the mean path; that is,Athe radius of the centroid is the radius -which divides the ow pathinto two portions of equal area.

asienta The radius Roof the centroid at the inlet may be determined forthis purpose from the dimensions of an inducer passage, fixed by theinner and outer radii of the inlet and the number and thickness of theblades, by graphical methods. The tangential velocity Uc of the impellerat the centroid is the product of Rc in feet by the angular velocity ofthe rotor in radians per second. The air velocity, assumed parallel tothe axis, equals the air tlow in weight units divided by the product ofthe air density and the total area of the inlet annulus (the inlet 19)and is represented by V1.

Given these values, the relative velocity of the air with respect to therotor, Vr, may be determined as indicated by the Vector diagram ofFigure 4. Experience has shown that an angle of attack of approximately10 degrees at the inducer entrance will give peak eiiiciency, duepresumably to the resulting increase in eective inlet area. Too great anangle of attack will result in stalling at the entrance.

The angle Q1 (Figs. 3 and 4) of the inducer blades at entrance istherefore taken as ten degrees less than the angle of Vr at the centroidto the axis. Since the blade equation is where XT is the known length ofthe inducer. This equation may be solved for K, which determines theform of the inducer blading. It may be noted that, since the blades areradial, the angle Q and the angle of attack vary with the radius.

So much having been done, it is now in order to begin the determinationof the mean path through the rotor and from the mean path the inner andouter boundaries of the path of air through the rotor.

The first stage in this operation is to prepare a plot of componentvelocity against path length, as illustrated in Figure 6. The pathlength is the length of the projection on a plane containing the rotoraxis of the path through the entire rotor of a particle of air enteringthe compressor at the centroid of the inlet, and must be estimated fromthe dimensions of the rotor.

The component velocity Vc is velocity in a plane containing the rotoraxis. In other words, component velocity is the resultant of the axialand radial components of the absolute velocity of a particle. Thisvelocity at inlet and outlet is likewise computed from the stated designconditions. Component velocity at Zero path length is that due to totalarea of the inlet. Component velocity at one halt` inch from the inletis increased by the decrease in path area due to the inducer blades, andmay be computed from total llow and net area of the path (static densityassumed to equal that at the inlet). From the one half inch point,component velocity is assumed to increase linearly with path length. Y

Figure 5 illustrates the relationship between U, tangential velocity ofthe impeller, Vr, velocity of the air relative to the impeller, and V,absolute velocity of the air, which is the resultant of U and V1. Italso illustrates the rectangular components Vc and V1, of V takenrespectively in a plane containing the axis of the impeller and normalto this plane. Vc is the component velocity and V1 the tangentialVelocity of the air.

It will be noted that the design assumption of linear increase incomponent velocity is based upon the desired result of smooth continuouschanges in Vc, a condition believed to promote efficiency ofcompression. The ultimate form of the rotor will be such as to justifythis assumption.

The principle underlying the construction of the optimum centroid of themean air liow path is illustrated in Figure 7, which showsdiagrammatically a single-sided impeller with axis of rotation X-X. Thecentroid of the air path, to be determined, is C-C. Assuming anelementary particle of air at a point P on the centroid, this particlewill be subject to a centrifugal force F1 due t0 its rotation about therotor axis. F1 forunit mass will equal tangential velocity VT squareddivided by the radius R1. The radius is defined by the position of P andequals O1P. The tangential velocity may be calculated. As indicated inFigure 5, the tangential velocity equals U Vc tan Q. V is the product ofR1 and the angular velocity of the rotor in radians per second. Vc maybe taken from the chart (Fig. 6). Q is determined by the form of theinducer blading and the radial and axial coordinates of point P.

The air iiow path obviously curves outwardly, the problem being todefine the path of natural iiow of the air so as to form the impeller toavoid dellecting the air from this path. Also, the curve C-C willclearly be a continuous curve. It will, therefore, have a center ofcurvature corresponding to the point P, indicated on the diagram at O2.The location of O2 is xed by the form of the curve C C and the locationof P. The elementary particle of air at P is rotating about theinstantaneous center O2 at a radius R2 equal to the radius of curvatureof C-C at point P. The velocity of P in this path is Vc, the componentvelocity (Figs. 5 and 6). The particle will therefore be subject to acentrifugal force F2 directed away from O2, the magnitude of which, forunit mass, equals VGZ/R2.

The resultant of F1 and F2. is indicated by F. If F is tangent to C-C atP, the resultant force on the particle acts along the centroid and thereis no tendency for the particle to `depart from the assumed centroid. Itthe curvature of the centroid is too great, the resultant F will bedirected inwardly from the tangent to C-C, and the actual air ow in therotor will crowd against the inner boundary of the flow path.Conversely, if the curvature is too small, the air liow will trendtoward the outer boundary of the rotor.

The centroid must therefore be determined in accordance with theprinciple illustrated by Figure 7. Conceivably, this problem might besolved by deriving an equation deiining the locus C-C. However, themathematical difliculties appear to be insuperable in View of thecomplication of the problem by such factors as friction, changingdensity of the air, and varying area of the liow path.

Various step-bystep methods of computation of the centroid may beemployed. I presently prefer a partly graphical method, illustrated inFigure 8, in which the centroid is constructed by drawing short circulararcs about assumed centers. The resulting values for each increment arechecked, and the assumed center varied, until the results areconsistent. By repeating this process, the entire curve is constructed.

Referring again to Figure 7, since F2 is normal to the centroid and Fshould be tangent to the centroid, the desired conditions will resultonly when F is normal to F2. The component of F1 normal to F musttherefore equal F2. This component equals F1 `cos B, where B is theangle between F1 and R2, which obviously equals the angle between thetangent to `C-C at P and the axis X-X.

Substituting in the equation F1 cos B=F2 the values of F1 and F2previously stated, and solving for R2, we have R2=Vc2R1/VT2 COS B.

Referring to Figure 8, the boundaries of the inlet, the centroid of theinlet, the axis of rotation X-X, and the split line between `the inducerand impeller are indicated. These are laid out, to any suitable scale,for the solution of the problem. The radius of the inlet centroid may becalculated by graphical methods :from the physical dimensions and lformof the impeller.

The ilow is assumed to have no radial component for the iirst half inchof the path (in a particular large impeller), an assumption whichintroduces no significant error. Thus, the first half inch is laid outas a straight line from the centroid of the inlet parallel to the axisto the point A. At this point, R1 is known, angle B is zero, Vc may betaken from the chart (Fig. 5), and VT is derived aan state 7 from Weland -the values -of U and .Q for the coordinates of point A, asindicated in .Figure .5.

Aichart showing thevalues oftan 1Q in terms of X and R throughout 4the-induoer `may be 'prepared for -ready reference. Tan Q-is kunity inthe-imp'ellen ilor the specific inducer described, tan Q is propontional tothe'product of X .and

The nvalue ot R2 'may thus be computed, and is laid out as line .AGnormal to the axis in Figure 8. With lO `as center, an arc is drawn toA. The extent of .this must be small, 'as Athe .computation `'proceedsby small increments. arc is exaggerated .in Figure v8 for clarity. Anarc of the order yof 2 degrees might Vbe used. Ol vionsly, .the arcmustapproximate.thesegment of'ftheinon- `circular locus which itrepresents.

Tlhe Acomputed 'rva'lue :of R2 eis .checked tor the interval AA bycalculating R2 ffor the .midpoint P of the arc. The .radial and axiadcoordinates of .and the value `ot angle B at .P are'determined in theobvious manner, yand the basic equation diz: VCZRI/:VTZ cosiB is solvedfor the 'valuesof Vc and 'VT for thefactual coordinates of?. This value-for radius R2, indicated by :the dotted line OP, should .closely.approach :the radius 0A. df .the discrepancy is ,greater than 'onepercent, .successive approximations may be made. Thus, if OP is greaterthan OA, a greater value of R220/l is assumed, a Anew arc is drawn,andthe valueoilz is recomputed 4for the neu/coordinates of P. When theequation :for the midpoint of the arc is balanced within one percent,the arc AA .may be iconsidered a sufficiently exact approximation tolthe .segment ot the desired centroid.

The vprocess yis repeated, computing R2=0A from the coordinates .of Aland the :value of angle B, Vc, and VT at A. The arc AA is drawn withcenter 0 on .OA' and radius equal to the new value of R2. The'midpointof this arc is checked and OA moditiedif necessary, thus .deter` yminingsuccessive radii R2, R2', R2, and so on. In this Way, a closeapproximation to the mathematically exact centroid vis built up segmentby segment, and the nal curve may appear substantially .as vindicated inFigure 7 by C-C.

When this has been done, Lthe length of -the centroid C-C is measured.l-f this varies signiticantlyzfrom the -value assumed, which is.thevztbscissa of the chart (Fig. 6) -of component velocity, a new.assumption as to length .of C-C is made, .the .chart is corrected, andthe centroid is recomputed. A discrepancy 'of one percent in the pathylengthis acceptable.

The iinal operation in dening the zforrn ofthe ,rotor consists ofdetermining the inner and outer :boundaries of the flow path, based uponthe centroid :as determined.

lf, as illustrated in Figure y9, we draw a normal .to .the .centroid atany point yP, and rotate the centroid and the normal about the axis ofrotation .of the vimpeller, the centroid will generatea complex .surface.ofsrevolution and .the normal will generate a conical surface. The area4of .the conical surface between the inner and outer boundaries of theair path through the rotor is taken as the total area AT or" .the towpath. The net area AN oit .the 4ilow path equals AT-./l, where Av is:the .portion .of AT .occupied by the vanos.

The desired valueof AN for any point on the centroid is derived asfollows: A curve offstatic density of the air against path length -isplotted, as illustrated in Figure 6. Static y.density at vthe `inletVand outlet are computed from the stated design conditions. Staticdensity is assumed to remain constant for the tirst .half inh of thepath (no compression as the air enters the spaces between the inducerblades) and then to increase .linearly .to the outlet. y

The desired net area AN of the patlrat anypoint is equal to the air flow(pounds per second) dividedbythe product of static density and.component velocity. AN .is computed and plotted against ,path length,Yas .indicated .in 'Figure 6.

tof .the blades in the inducer.

cite calculations based upon specilic values The flocation of the innerand outer envelopes o 'the flow path is determined so `that ltheportions -o'f the ne't flow -path outside 1and inside the `centroid C--Care vvof equal area. The desired area Imay be obtained by properl-yflo'cating theseenyelopes. Since AT, AN, and Av-do not vary in the sameIratio with changes inthe depthof the air passage, and no simplerelation exists l'between AT, AN, and Av, a method Aof successiveapproximations is used.

Referring to Figure 9, the ycentroid C--C has beenfestablished. Todetermine the inner and outer boundaries of the l'low path, shown inbroken lines, a series of points on leach boundary are found, each pointcorresponding to `a'point on A`the-centroid. With a number of pointsthus iocated, curves may be aired through these points.

For any Vpoint "P, Ifor example, 'the desired =value of AN at Pisdeterminedfrom fthe curve (Fig. 6) for `'the distance of point TP fromtheinlet along the line C-C.

The value of AT A'sufficient -to give the l'required AN is estimated.'The fangle TB between the 'radius OP l'and the normal to the centroidis measured.

The values of L1=PP and L2=PP corresponding Lto the assumed value of ATare determined. The areas of the conical surfaces `generated by rotationof these line segments about the axis X-X must each equal one-half AT.

Therefore, from the .geometry :of yFigure 9,

1,2 .AT cos B The -total width of the passage, which `equals the depth.ofthe Iblade, 4is LVI-L2, which we ymay call L. Then AT=frL(R3i-R4f),which relation may be used ato check .the computation.

Since AN=ATAW Av must be determined. Av maybe .found with sufficientaccuracy `from `the formula A=NLt/sin Q where .N .is .the number ofvanes, .t is Ithe .average thick- -ncss of the vaues, .and Q .is taken.as the value .at the centroid. The term -sin Q corrects for theinclination ln the impeller, sin Q equals unity.

The value of AN=AT-A obtained by subtraction is checked against lthevalue .of AN lorginally assumed. If

the .discrepancyis greater than one percent, the assumed value of AT .isadjusted, and L1, L2, Av, and AN are re* computed until a satisfactoryYagreement is reached.

A series of points .on the inner and outer boundaries -ot the bladesbeing thus determined, .the form of therotor 'is completely established.The casing .'21 is formed 4rfor a small clearance from the rotor.

The .rotormush of course, be Aanalyzed .for centrifugal vand otherstresses, but this procedure is .not material to my `invention and neednot be .explained lherein.

The .foregoing explanation and description ,will enable those .skilled.in the art to practice the invention. It is unnecessary to anunderstanding of the invention to reof design parameters or to specifythe resulting forms and dimensions; each compressor according to theinvention must be designed for its` particular environment.

It will be seen from .the foregoing that the .salient feature .ofcompressors accordingto my invention is ,that .the

radial component .of velocity oftheair `tlow is'developed 9 or, in otherwords, the airis dee'ct'ed* from axial flow to iiow with a substantialradial component, by the free action of centrifugal force.

This distinguishes from previous compressors in which the deflection ofthe air flow from substantially axial to more or less preponderantlyradial is either forced by the inner boundary of the iiow path orrestrained by the outer boundary. In such prior compressors, the forceddeiiection or forced restraint -of deflection results in unequalpressure and velocity distribution across the flow path, reducing thetotal iiow, pressure ratio, and etiiciency of the compressor.

' The compressor of the invention may be distinguished from prior artcompressors by the term free deflection compressor, and the principle ofthe invention may be called the free deflection principle.

An example illustrating the benefits of `the invention may be cited. Thecompressor of a previously existing gas turbine engine has beenredesigned in accordance with this invention. The new compressor rotoris equal in diameter to the previous rotor. Notwithstanding this,striking improvements in the performance of the engine have beenrealized. The installation comprises a rotor of fifteen inch radiusdirectly coupled to a turbine. The air flow has been increasedtwenty-two percent, the pressure ratio has been raised from 4.4 to 4.9,and the efciency of the compressor from the former seventy-four percentto eighty percent. This improvement could not have been achieved byapplying previously known principles of compressor design.

As a result, the specific thrust of the engine in pounds thrust perpound of air has Abeen increased fteen percent and the fuel consumptionfor unit thrust has been reduced by twelve percent. It may be noted thatthese gains vinvolved an increase of 1e`ss than one half of one percentin compressor R. P. M. The turbine of the engine was. redesigned to litthe increased air ilow, but the improvement of the engine is not basedupon increase in turbine eiciency.

Since this degree of improvement was made in a compressor and an enginewhich had been highly developed for some years prior to the invention,the importance of the invention will be obvious.

The above `description relates to a compressor in which componentvelocity increases linearly with path length. Obviously, otherassumptions as to component velocity could be made, and the principlesdescribed are applicable to the design and construction of compressorsbased on any `desired relation between component velocity and pathlength.

It will be apparent to -those skilled in the art that my invention maybe practiced in a variety of forms and that the principles thereof arereadily capable of wide application. The invention is not to beconsidered as limited by the detailed description of an illustrativeembodiment thereof.

I claim:

1. A centrifugal compressor comprising a vaned rotor rotatable about anaxis, means deining an inlet to the rotor for entrance of air axially ofthe rotor, and means dening a diffuser for air discharged from theoutlet of the rotor, the rotor being formed to discharge air radiallyrelative to the rotor, the vanes of the rotor defining an air ow pathfrom inlet to outlet the area of which varies from inlet to outlet toprovide an even variation in air velocity along the path, the interceptof the mean surface of the path by a plane containing the axis ofrotation being tangent to the resultant of the centrifugal force vectorsdue to tangential velocity of the air and to velocity of the air in thepath defined by the said intercept of the mean surface at each pointalong the said mean surface.

2. A centrifugal compressor comprising a vaned rotor rotatable about anaxis, means defining an inlet to the rotor for entrance of air axiallyof the rotor, and means defining a diluser for air discharged 'froml theoutlet o'f the rotor, the rotor being formed to discharge air radiallyrelative to the rotor, the vanes of the rotor dening an air flow pathfrom inlet to outlet the intercept of the mean surface of which by aplane containing the axis of rotation is tangent to the resultant of thecentrifugal force vectors due to tangential velocity of the air and tovelocity of the air in the path defined by the said intercept of themean surface at each point along the said mean surface.

3. A centrifugal compressor of the axialto-radialflow type comprising,in combination, means defining an annular inlet and a vaned rotorcomprising an inducer portion adapted to accelerate a gas streamcircumferentially relative to the rotor axis `and an impeller portionadapted to accelerate the gas stream radially and tangentially, the gasiiow path between the rotor vanes varying from inlet to outlet of therotor so as to provide substantially linear variation in the resultantof the axial and radialfco-mponents of gas velocity, the inner and outerboundaries of the gas iiow path through the rotor being so disposed thatthe centroid of the path substantially coincides with the freedeflection path of gas ow through the rotor under design conditions.

4. A centrifugal compressor comprising a vaned rotor with an inducerportion and an impeller portion7 the mean ow path through the rotorconforming substantially to that defined, for design conditions ofoperation, by the following procedure: Assumption of substantiallylinear increase through the rotor of the component of velocity of theair in a plane containing the rotor axis; and derivation of a ilow pathoriginating at the centroid of the inlet and corresponding to the freepath of flow of a particle under the effect of centrifugal force andwith the component of velocity varying as stated.

5. A centrifugal compressor comprising a vaned rotor with an inducerportion and an impeller portion, the boundaries of the ilow path throughthe rotor conform ing substantially to those defined, for designconditions of operation, by the following procedure: assumption ofsubstantially linear increase through the rotor of the component ofvelocity of the air in a plane containing the rotor axis; derivation ofa centroid of the flow path originating at the centroid of the inlet andcorresponding to the free path of flow of a particle under the effect ofcentrifugal force and with the component of velocity varying as stated;assumption of substantially linear increase through the rotor of airdensity; determination of the required net area of the path as afunction of path length for component velocity and density varying asstated; and location of the inner and outer boundaries of the flow pathso that the path is divided into two equal parts by the centroid and thenet area of the path conforms to that determined as stated above.

6. A centrifugal compressor of thef."azrial-tofradiahiiow typecomprising, in combination, means defining an annular inlet and a vanedrotor comprising an inducer portion, with vanes decreasing progressivelyin angular relation to the axis of the rotor from a maximum angle at theinlet to substantially a zero angie at. the discharge end of theinducer, and an impeller portion with vanes substantially at a zeroangle to the axis, the axial length of the inducer being such that thetangential acceleration of gas flowing through the inducer accords withthe tangential acceleration of the gas in the portion of the impelleradjacent the inducer.

7. A centrifugal compressor of the axial-to-radialiiow type comprising,in combination, means defining an annular inlet and a Vaned rotorcomprising an inducer portion, with vanes decreasing progressively inangular relation to the axis of the rotor 'from a maximum angle at theinlet to substantially a zero angle at the discharge end of the inducer,the vanes being curved to provide substantially uniform tangentialacceleration of gas in the inducer, and an impeller portion with vanessubstantially 1'1 yat aizero angle `to s-the axis, the axial lengthefthe Yinducer being such that :the tangential acceleration of gasEtion/ing :through the inducer raccords with the vtangentialacceleration Lof 'the .gas in fthe-portion of the impeller adjacent theinducer.

`8. A compressor comprising, in combination, `a rotor rotatable about anaxis, .means defining an annular inlet for directinggas into the rotorsubstantially parallel -to .thelaxis, and a `diuserfor receiving lgasfdischargedifrorn the rotor, the rotor comprising a body in the form lof.-a 'body yof revolution about the laxis with vanos extending from thesurface tof vlthe -body Asubstantially normally to ythe surface, thespaces bet-Ween adjacent Vanes constituting ygas flow paths from the`inlet y'to the diffuser trending Asmoothly .from a direction:substantially parallel ,to the la-Xisat the inlet ends thereof toVa'direction approximately normal to the :axis at lthe discharge endsthereof, the depth 'of the 4Yanes r:varying from inlet to youtlet sothat lthe area of each fpath .varies `'as =a @function of Vdistancealong .the path `accordiugito the relation .that the .area .-is thequotient y,of a constant by ithe :productloftwo quantities each vary ingsubstantially linearly with distancetal'ongtthe path, the saidquantities being component velocity yof gas `in the path and staticdensity of gas in the path.

V9. A compressor comprising, 1in combination, a rotor rotatable about anaxis, tmeans defining an annular inlet for directing .gas into Vtherotor substantially parallel -to .the axis, and a diiuser .for receivinggas discharged `from the rotor, the rotor `comprising .a Abody 4in the.'forrn of a 'body of revolution 'about the axis 4.with vanes extendingfrom vthe :surface of the body substantially normally Vto the surface,the spaces between nadjacent vanes constituting gas flow paths -from theinlet to tthe diffuser trending smoothly from a direction substantially:parallel :to the axis at the inlet ends thereof toadirectionapproximately normal to the 'axis at -the discharge ends`thereof, Vthe form of the surface ofthe body `being such and the depthof the vanes varying from :inlet to outlet so as :to :define a path thelocus of 1the geometrical center/of which is tan- `l2 gent at anyiPOlKlt along :the ,path to tthe resultant of the centrifugal `force`Vectors at that point due v.to .tangential ne locity of the gas aboutthe said anis Yand to ,yeloeitytofth gas yinthe saidfpath.

L0. yA -Xcompressor as recited in claim 9 in ywhich the area Yet thepath varies ,with Idistance :along 4the `path ,so that .the velocity fofthe gas .in .the path varies linearly with distancealong the path.

l-l.. lA compressor compri-sing, 4in combination, a ,rotor rotatableabout .an axis, means .defining ,an annular inlet for directing gas intothe rotor .substantially parallel to the axis, and a diffuser forreceiving gas discharged `from the rotor, the rotor `rcomprising a .bodyin .the form `of a body .ofrevolution I.about the axis with yanesextending from the surface ofthe `bodysubstantially normally @to ,thesurface, the spaces vbetween .adjacent Names constituting gas 4How pathsfrom l,the inlet tto .the `diffuser trending smoothly from .a .direction,substantially parallel to the axis at the .inlet ends thereofto.aldireetionapproximately normal tothevaxis at ,the `discharge endsthereof, theorm of the surface lof ythe body lbeing such and the depthof the yYanes varying ,from inlet :to outlet .so .as .to define a path:the locus ofthe geometnicalfcenter `of whichlis the free deflectionpath foriflow ofgas :from the inlet to the outlet,

References Cited 4in vthe lfile of this ypatent UNITED STIATESk,BIATENTS A1,097,729 Rice May 26, 1914 2,228,194 Birkigt Jan. 7, v1941'2,398,203 Browne Apr. 9, 1946 2,899,852 -Campbellfet al May 57, 1946FOREIGN vI JATENTS 279,426 :Great Britain ;Aug. 9, 51928 l460,985'Germany June 8, 1928 Sil-2,098 AGreat Britain .of 1939 6119,617 l"GreatBritain kof 1:949

